This invention relates to a method of, and apparatus for, controlling the advance of a tunnel drive shield, and to a drive shield incorporating such apparatus. The term "tunnel" or "tunnels"]used throughout this specification and claims is intended to include galleries, trenches, adits or other similar elongated excavations.
It is known to use drive shields for excavating tunnels in ground having widely varying properties. When the ground being excavated is rock, tunnels are driven with the help of a special rock breaking device. However, even rocky ground is by no means of uniform consistency as it is often interlayered with sandy ground so that there always exists the danger that, unless the walls of the excavation are immediately supported, they will cave-in. Consequently, it is important to support tunnel walls immediately after they have been excavated.
One known drive shield which does just this is a knife shield, that is to say a shield of the type having a plurality of elongate implements (or planks), each of which is supported and guided on a frame. The planks are movable relative to the frame by means of hydraulic rams. Thus, immediately a tunnel is excavated, the planks can be moved up, either singly or in groups, to support the walls of the tunnel.
A disadvantage of this type of shield is that the known methods of advancing it in a predetermined direction rely on a large contact area between the shield and the tunnel walls. This is because the large forces which are necessary to advance the shield have to be taken up entirely by the frictional force between the shield and the tunnel wall. Where the ground being excavated is rock intercalated with sand, adquate contact between the shield and the tunnel walls will not always exist.